The composition of fossil fuel-derived flue gases is commonly made up of carbon dioxide (CO2), nitrogen (N2), oxygen (O2), sulfur dioxide (SO2), nitrogen dioxide (NO2), and fly ash of inorganic oxides such as SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O, K2O and P2O5.1 In order to comply with environmental regulations, the flue gas streams must be treated prior to atmospheric discharge to remove CO2 known as a major greenhouse gas. Absorption via chemical reactions using aqueous amine solutions has been found to be a very effective technique in separating CO2 from these low pressure flue gas streams in order to reduce CO2 to the required concentration target. However, reactive contaminants, specifically O2 and SO2, constantly introduce a serious amine degradation problem during the absorption process. Initially, inside the absorber, O2 and SO2 make first contact with the amine solution during the CO2 absorption process causing amine breakdown. The structural deterioration is then aggravated in the regenerator where a combination of high temperature and highly soluble SO2 carried over with the liquid stream promotes additional amine degradation. The degradation severely affects the absorption plant by reducing the CO2 absorption capacity of the amine and inducing corrosion and foaming problems due to accumulation of the degradation products.
The literature has suggested a long-term solution for amine degradation by locating the source of O2.2 However, this approach is highly complicated since a complete removal of O2 as well as SO2 from a major source such as flue gas streams is impractical. Also, the detection and removal of O2 is especially known to be time-consuming and labor-intensive.3 To maintain the performance of the CO2 absorption process, a quick prevention technique must then be included as one of the operational routines of the absorption unit to immediately cope with the degradation problem. One of the preferred choices is the use of degradation inhibitors due to their simplicity and instant effect.
Consequent upon the test and confirmation of the severity of oxidative degradation of various alkanolamines, including monoethanolamine (MEA), diethanolamine (DEA) and methyldiethanolamine (MDEA), it was recommended by Rooney et al., and Rooney and Dupart to use O2 scavengers such as sulfites, hydroxylamine, and hydrazine (that are typically used in boiler-feed water) to reduce O2 to ppm levels in alkanolamine systems.2,4 Useful guidelines for selection of appropriate inhibitors were later given in the literature.5 According to Veldman, O2 concentration in an alkanolamine unit was normally low, thus only allowing the reaction to proceed as a partial oxidation reaction to produce carboxylic acids rather than a full oxidation of the alkanolamine to CO2 plus NO2.5 Their DEA degradation experiments showed that partial oxidation of DEA to carboxylic acids proceeded at temperatures of less than 323 K with dissolved O2 of less than 1 ppm in the solution.
The use of a commercial corrosion inhibitor which also acted as O2 scavenger was reported to control the level of bis(2-hydroxyethyl)glycine (bicine), an oxidative degradation product in a commercial MDEA-based gas treating unit.6 To effectively control O2 from degrading MDEA to bicine, scavenging O2 in the liquid phase of alkanolamine solution was preferred as compared to the gas phase of flue gas streams. With this approach, the rate of bicine build-up in MDEA solution in an industrial plant was reduced from 60 ppm/day to 6 ppm/day. Unfortunately, the inhibitor/O2 scavenger information was not disclosed in the literature.
Chi and Rochelle investigated various additives as potential degradation inhibitors in iron catalyzed MEA oxidative degradation system with and without CO2.7 The additives consisted of ethylenediaminetetraacetic acid (EDTA), bicine, glycine, and diethylethanolamine (DEMEA). Only EDTA and bicine were reported to be effective in reducing the degradation rate of MEA. EDTA was found to decrease the rate of oxidation of MEA when CO2 was present. However, it had no effect when CO2 was absent from the oxidation system. Bicine, a degradation product itself, was also found to be effective in reducing the MEA oxidative degradation rate.6 It decreased the degradation in systems with and without CO2. It must be noted that a contradictory result was reported by this study in terms of CO2 loading effect in MEA degradation systems. An increase in the CO2 loading was found to increase the degradation rate. This result was opposite to those reported in other works.2,8,9,10,11 
Recently, degradation inhibitors for copper and iron catalyzed oxidative degradation of MEA have been evaluated.12 Various compounds including undisclosed inorganic Inhibitor A, sodium sulfite (Na2SO3) and formaldehyde were investigated. The experiments were all carried out using conditions corresponding to the top and bottom of the absorber with 7 kmol/m3 MEA, air containing 21% O2, lean/rich CO2 loading, and at 328 K. Inhibitor A was found to successfully reduce MEA oxidation rate in both copper and iron catalyzed systems. It was also effective in the systems with lean and rich CO2 conditions. Inhibitor A was also found to inhibit the MEA degradation more easily in Cu catalyzed system and rich CO2 loading than in Fe catalyzed and lean CO2 loading environments. Na2SO3 also decreased MEA degradation rate in both copper and iron catalyzed systems. For copper-catalyzed MEA degradation, Na2SO3 decreased the degradation rate until its concentration reached 100 ppm. The degradation rate was found to increase if a higher concentration was used. Although, formaldehyde could reduce the degradation rate, it was not as effective as Inhibitor A. Both Na2SO3 and formaldehyde worked more effectively in copper catalyzed environment than iron system. As opposed to Na2SO3 and formaldehyde which were also found to deteriorate during MEA degradation, Inhibitor A did not decompose. Inhibitor A therefore, did not need to be further added or replaced later in the process.